solved 'Reorder List' on LeetCode (ONE AC! oh yeah;) http://oj.leetcode.com/problems/reorder-list/

ReorderList.java

/**
 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
        public void reorderList(ListNode head) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        if (head == null) return;
        List<ListNode> l = new ArrayList<ListNode>();
        ListNode iter = head.next;
        while (iter != null) {
            l.add(iter);
            iter = iter.next;
        }
        iter = head;
        int i = l.size()-1;
        while (i>=0 && l.get(i) != iter) {
            ListNode tmp = iter.next;
            iter.next = l.get(i);
            if (iter.next == tmp) {
                i = 0; // break
            }
            else {
                iter.next.next = tmp;
            }
            iter = tmp;
            i--;
        }
        iter.next = null;
    }
}